There is known a device for controlling an electric field at a high voltage component comprising an inner deflector to be electrically connected to a live high voltage part of the high voltage component; a resistive layer adapted for field controlling purposes, to be arranged along the high voltage component, and which at one position is electrically connected to the inner deflector and at one end is adapted to be electrically connected to a grounded part of the high voltage component; an insulating layer arranged on the resistive layer and extending at least from the one position towards the one end of the resistive layer while ending without reaching the one end of the resistive layer; and a semi-conducting layer arranged on the insulating layer and extending at least from the one position towards the one end of the resistive layer and past the end of the insulating layer, thereby defining an outer triple point at the intersection of the resistive layer, the insulating layer, and the semi-conducting layer.
A device of this type is known through for instance WO 00/74191 A1. A device of this type is used for distributing the electric field where this is crucial and by that avoiding electric field concentrations and damaging of different type of equipment, such as cables. The electric potential between the live part and ground is distributed by means of a material with a suitable resistance forming the resistive layer. By combining the resistive field control and a suitable geometrical field control obtained through the insulating layer the risk of harmful charges building up and of high stresses upon the component, such as a cable, arising at rapid changes of the voltage thereof, is reduced with respect to such a device having only a resistive field control through a the resistive layer.
EP1870975 A discloses also a device of this type for controlling an electric field at a high voltage component, the device comprising a resistive layer for field control, an insulating layer arranged on the resistive layer and a semi-conducting or conducting layer arranged on the insulating layer. The three layers meet at a triple point, where the insulating layer ends. The interface between the resistive layer and the insulating layer makes in the triple point an angle to the semi-conducting or conducting layer of 60°-120°.